Rotating and pivoting magnet for magnetic navigation

ABSTRACT

A magnet assembly comprising a magnet mounted for pivoting about a first axis spaced from the magnet, and rotating about a second axis that is perpendicular to and intersects with the first axis. The magnet comprising a plurality of segments each with a magnetization direction such that through a combination of pivoting and rotating the magnet projects a magnetic field in any direction at an operating point spaced from the front of the assembly. The segmented construction with segments of different magnetization directions allows small changes in the orientation of the magnet to substantially change the magnet field direction at a system operating point.

FIELD OF THE INVENTION

[0001] This invention relates to magnet medical procedures, and inparticular to a magnet useful in navigating magnetic medical devices inthe body.

BACKGROUND OF THE INVENTION

[0002] Electromagnets and permanent magnets have been developed formoving magnet medical devices in the body. Some magnets used in medicalapplications apply a gradient to pull magnet medical devices within thebody. Other magnets used in medical applications simply apply a magneticfield in a selected direction to align magnetic medical devices in theselected direction. Still other magnets apply both a magnetic field anda magnetic gradient to simultaneously orient and move a magnetic medicaldevice.

[0003] There are a number of important competing design considerationsfor magnets used in medical procedures. First and foremost is providingsufficient field strength or gradient to orient or move the magneticdevice. Electromagnets and in particular superconducting electromagnetscan create strong magnet fields and gradients, but they are expensive toconstruct and operate. Until recently, it was difficult to construct apermanent magnet that could provide a sufficiently strong anduniversally directed magnetic field and gradient at a distancesufficiently far from the magnet to be useful in medical procedures.Recently, a focused permanent magnet has been developed which can createuseful magnet fields at sufficient distances from the magnet to beemployed in magnet surgery. The magnet is comprised of a plurality ofsegments each magnetized in a direction to contribute to the desiredmagnetic property, for example field strength at an operating pointspaced in front of a magnet. This magnet and its method of design aredisclosed in copending, co-owned, U.S. patent application Ser. No.09/546,840, filed Apr. 11, 2000, U.S. patent application Ser. No.09/497,467, filed Feb. 3, 2000, the disclosures of which areincorporated herein by reference. This magnet has other usefulproperties in that field direction could be changed by a simpletranslation of the magnet. However, these magnets still had relativelylarge exclusion zones to accommodate the movement of the magnet. Thelarge exclusion zone made access to the patient, and positioning ofother medical equipment (particularly imaging equipment) in theprocedure room difficult. Thus a second design criteria is to minimizethe exclusion zone, to provide greater access to the patient for medicalstaff and equipment.

[0004] A third design criteria is to minimize the degrees of freedom ofmagnet motion to provide a universally directed magnetic field. Thefewer degrees of freedom of magnet motion needed, the simpler thenavigation, and the less expensive the apparatus for moving the magnet.

SUMMARY OF THE INVENTION

[0005] The present invention relates to a magnet, and to a magnet systemthat is capable of generating useful magnet fields in virtually anydirection, at distances from the magnet sufficient to conduct medicalprocedures in the patient's body. The magnet is designed so that amagnetic field can be generated in virtually any direction with aminimum amount of movement so that the exclusion zone—the zone fromwhich the patient and other medical equipment and personnel cannot belocated—or the inclusion zone—the zone that the magnet occupies—isminimized.

[0006] Generally the magnet of the present invention comprises aplurality of magnet segments each magnetized in direction to optimizethe magnetic field at an operating point spaced from the magnet. Themagnet is adapted to pivot about a first axis spaced behind the magnet,and to rotate about a generally horizontal axis. Through a combinationof pivoting and rotating the magnet can project a magnetic field at theoperating point in virtually any direction of sufficient strength to beuseful. The shape of the magnet is determined to minimize the inclusionzone, which in the preferred embodiment is a horizontal cylinder, with abeveled edges on the forward face.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1A is a front elevation view of a magnet constructedaccording to the principles of this invention;

[0008]FIG. 1B is a right side elevation view of the magnet;

[0009]FIG. 1C is a top plan view thereof;

[0010]FIG. 1D is a front perspective view thereof;

[0011]FIG. 1E is a rear perspective view thereof;

[0012]FIG. 2A is a perspective view of a support for pivoting androtating a magnets in accordance with the principles of this invention,with the magnet in a first position;

[0013]FIG. 2B is a perspective view of a support for pivoting androtating a magnets in accordance with the principles of this invention,with the magnet pivoted to a second position;

[0014]FIG. 3A is a perspective view of a housing containing the magnetand support;

[0015]FIG. 3B is a front elevation view of the housing;

[0016]FIG. 3C is a right side elevation view of the housing;

[0017]FIG. 3D is a top plan view of the housing;

[0018]FIG. 4A is a perspective view of one quadrant of a magnet block,with several surfaces of equal contribution (represented in wire frame)superposed thereon;

[0019]FIG. 4B is a top plan view of one quadrant of a magnet block withseveral surfaces of equal contribution;

[0020]FIG. 4C is a right side elevation view of one quadrant of a magnetblock with several surfaces of equal contribution;

[0021]FIG. 4D is a rear elevation view of one quadrant of a magnet blockwith several surface of equal contribution.;

[0022]FIG. 5 is a perspective view of the inclusion volume of a magnetconstructed according to the principles of this invention, showing themagnet generally centered within the inclusion volume;

[0023]FIG. 6 is a front elevation view of the exclusion volume with themagnet in its centered position;

[0024]FIG. 7 is a right side elevation view of the exclusion volume withthe magnet in its centered position;

[0025]FIG. 8 is a top plan view of the exclusion volume with the magnetin its centered position;

[0026]FIG. 9 is a perspective view of the inclusion volume, with themagnet pivoted to the left about the z axis;

[0027]FIG. 10 is front elevation view of the inclusion volume of themagnet, with the magnet pivoted to the left;

[0028]FIG. 11 is a right side elevation view of the inclusion volume,with the magnet pivoted to the left;

[0029]FIG. 12 is a top plan view of the inclusion volume, with themagnet pivoted to the left;

[0030]FIG. 13 is a top plan view of a magnet constructed according tothe principles of this invention, showing the local magnetic fielddirections in the space surrounding the magnet;

[0031]FIG. 14 is a horizontal cross sectional view of one half of amagnet constructed according to the principles of this invention (theother half being a mirror image thereof), showing the magnetizationdirections of the segments comprising the magnet, and the local fielddirections surrounding the magnet and lines of constant magnetic fieldstrength;

[0032]FIG. 15 is a graph of maximum coning angle versus distance fromthe magnet;

[0033]FIG. 16 is a perspective view of a magnetic surgery systemincorporating a magnet constructed according to the principles of thisinvention; and

[0034]FIG. 17 is a perspective view of a magnetic surgery systemincorporating two magnets constructed according to the principles ofthis invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] A magnet constructed according to the principles of thisinvention is indicated generally as 20 in FIGS. 1A through 1E. Themagnet 20 comprises a generally cylindrical front face 22 and a backface 24. There are left top face 26 and a right top face 28, and a leftbottom face 30 and a right bottom face 32. The magnet 20 preferablycomprises a plurality of parallel bands or segments of permanentmagnetic material extending from top to bottom. The magnetizationdirection of each segment is preferably selected to generally optimizethe magnet field at a magnet operating point spaced from the center ofthe front face of the magnet. This magnet operating point is a designcriteria of the magnet. For applications where a magnet field is to beapplied relatively close to the magnet, such a neurology applications,the magnet operating point may be selected closer to the surface of themagnet, for applications where a magnetic field is to be appliedrelatively far from the magnet, such as cardiac applications, the magnetoperating point may be selected further from the surface of the magnet.In this preferred, embodiment the magnet operating point is 13 inchesfrom the center of the front face of the magnet. This represents areasonable compromise to provide a magnet useful for both neurology andcardiac applications. Of course, the magnet could be optimized for someother operating point closer to or further from the front face of themagnet.

[0036] The magnet 20 is preferably mounted for pivoting about a firstaxis A1, generally parallel to the vertical axis of the magnet. As shownin FIGS. 2A and 2B, upper and lower arms 34 and 36 project from the backsurface 24 of the magnet 20. A cylindrical post 38 extends between thearms 34 and 36, and is journaled in a sleeve 40. The magnet ispreferably also mounted for rotation about a second axis A2, that isgenerally horizontal, and that is perpendicular to, and intersects with,axis A1. As shown in FIGS. 2A and 2B, a sleeve 42 extendsperpendicularly to sleeve 40, and is journaled around a horizontal arbor44. Of course any other mechanism for mounting the magnet 20 to pivotabout a first axis, and rotate about a second axis, and in particular topivot about a first axis that rotates about a second axis can be used.In the preferred embodiment the axis A1 is fifteen inches from the frontface of the magnet 20

[0037] A housing 50 for containing the magnet and structure for pivotingand rotating the magnet is shown in FIGS. 3A through 3D. The housing 50contains the magnet and mechanism so that it is isolated from theprocedure. Furthermore, the housing 50 eliminates moving parts from theprocedure site, so that the system is less intimidating to the patients,and does not present any hazard to anyone at the procedure site. Thehousing 50 accommodates the inclusion zone of the magnet 20.

[0038] As described above, the magnet 20 is adapted to pivot about anaxis A1 generally behind the magnet. The radius of curvature of thegenerally cylindrical front face 22 corresponds to the distance betweenthe front face and the pivot axis (15inches in this preferredembodiment). The back face of the magnet is shaped in accordance with asurface of constant contribution to the magnetic field at the operatingpoint. Material on such a surface contributes equally to the magneticfield at the operating point, regardless of its position on the surface.By selecting the appropriate surface of constant contribution to achievethe desired magnet size and strength, an excluding material that wouldlie beyond the surface, the weight of the magnet can be optimized forits selected magnetic properties. A constant contribution force can becalculated or plotted by maximizing the contribution to a particularmagnet property at the magnet's operating point, for example thetransfer field at the magnet's operating point, and determining thesurface of points that contribute equally to the selected magneticproperty. The superposition of several such surfaces df constantcontribution is shown in FIGS. 4A through 4D. As shown in FIGS. 4A to4D, various surfaces of constant contribution S₁, S₂, S₃, S₄, S₅, andS₆, are shown, and the final shape of back side of the magnet isdetermined based upon the constant contribution surface that leavessufficient magnetic material to achieve the desired field strength,gradient, or field gradient product, while keeping the weight low. It isdesirable to keep the weight of the final magnet low both to conservemagnetic material, which can be expensive, and to reduce the structuralrequirements for the supporting mechanism for the magnet. Because oflimitations of manufacturing magnets with smooth continuously curvedsurfaces, the actual shape of the back surface may only approximate theshape of the constant contribution surface. In the preferred embodiment,the magnet is capable of producing a field of at least about 0.4 T at anapplication point at least 13inches from the surface of the magnet, orabout 0.1 T at an application point of 7.5 inches from the surface ofthe magnet, yet weights less than about 500 pounds.

[0039] An important design criteria for the magnet 20 is its inclusionvolume, which represents the combination of all of the volumes that themagnet occupies throughout all of the desired possible orientations ofthe magnet, i.e., all of the desired pivots and rotations. The inclusionvolume of a magnet constructed according to the principles of thisinvention is shown in FIGS. 5 through 8, with the magnet in a firstposition within its exclusion zone, and in FIGS. 9 through 12 with themagnet 20 in a second position within its exclusion zone, pivoted 35°,which because of the design of the magnet described above, results in amagnetic field direction shift of 90° at the system's operation point.The system's operation point is a design element, and in this preferredembodiment is thirteen inches from the center of the front face of theinclusion volume, which corresponds to thirteen inches from the centerof the front face of the housing 50. The magnet's operation point andthe system's operation point correspond when the magnet 20 is in itscentered position in its exclusion zone. In the preferred embodiment thepivot point is 15 inches behind the front face of the magnet, and 28inches (15 plus 13 inches) behind the operating point. As shown anddescribed in the Figures, the pivot point is generally horizontal, andextends through the pivot axis. In this preferred embodiment, theinclusion volume is generally cylindrical, with a beveled forward edge.The inclusion volume has a diameter of about 30 inches and a depth ofabout 14 inches. The bevel on the forward face of the volume is atapproximately 45°, to a depth of about 5 inches, so that the diameter ofthe generally circular front face is about 20 inches. The edge of themagnet 20 is shaped so that the magnet 20 remains within the exclusionvolume.

[0040] Two magnets 20 can be mounted in opposition, so that theirmagnetic fields add, to provide a useful magnetic field at greaterdistances, for example to conduct cardiac procedures in the chest, wherethe application point of the magnetic field is necessarily far away fromthe magnet.

[0041] As shown in FIG. 13, when the magnet 20 is in its centeredposition, it produces a transverse magnetic field at an operating pointat the front of the magnet assembly. Rotation of the magnet 20approximately 35° clockwise about an axis parallel to the longitudinalaxes of the magnetic segments results in a magnetic field at theoperating point at the front of the magnet assembly to point outwardly,away from the magnetic assembly, and rotation of the magnetapproximately 35° counterclockwise about that axis results in a magneticfield at the operating point at the front of the magnet assembly topoint inwardly, into the magnetic assembly. Thus over the span of a mere70° of pivoting, the magnetic field direction changes 180°. Thispivoting, combined with rotation of the magnet about the second axis,allows the magnet to create a magnetic field in any direction at theoperating point of the assembly, through a simple pivoting and rotationof the magnet, without translation. Thus the inclusion volume of themagnet can be made very small, which means that exclusion volume issmall, and access to the patient by health care professionals andmedical equipment is not impaired.

[0042] While it is possible with the magnet assembly of the presentinvention to project a field at the application point in any direction,at sufficient strength to be useful, it may not always be possible tomove smoothly and continuously from one magnetic field direction toanother in the plane containing both directions. Thus when changing thefield from a first direction to the second direction, it is possiblethat a field direction will temporarily swing out of the plane—aphenomenon known as coning. However, amount of coning depends upon thedistance from the magnet, and as shown in FIG. 14, the maximum coning isslightly more than 14° from the desired plane, and occurs at distancesof about six inches from the magnet. At a distance of 12 inches from themagnet, the maximum coning is about 12.75°.

[0043] A magnetic surgery system incorporating a magnet systemconstructed according to the principles of the present invention isindicated generally as 100 in FIG. 16. The system 100 includes a magnet20 and its support and moving structure contained within a housing 50.The system 100 is particularly adapted for conduct neurologicalprocedures, and the housing is positioned to be near the patient's bead,in this case at the top of the patient's head. The system 100 includes apatient support, such a patient bed 102, which may or may not bemovable. A C-arm 104 mounts bi-planar imaging equipment for makingbi-planar images of the procedure site, and displaying them on thedisplays 106. The bi-planar image equipment includes an imaging beamsources, such as x-ray sources 108, and imaging beam receivers ordetectors, such as amorphous silicon last plates 110, which aresubstantially unaffected by the presence of magnetic fields. The magnet20 inside the housing 50 can be used to navigate a magnetic medicaldevice in the patients head by pivoting the magnet about axis A1androtating the magnet about axis A2 to achieve the desired magnetic fieldto orient a magnetic medical device inside the patient's head. Thebi-planar imaging allows the physician and other health care workers tomonitor the orientation and position of the magnetic medical device tonavigate the distal end of the magnetic medical device to its desireddestination. While the magnet assembly is designed to apply a magnetfield at the systems' operating point, which, as described above is apoint thirteen inches from the front face of the housing 50, the systempreferably allows the application of a magnetic field in virtually anydirection in sufficient strength for navigation purposes, e.g. 0.1 T,anywhere in 7 inch diameter cylinder surrounding the line from thecenter of the front face of the housing to the system's operating point.

[0044] A magnetic surgery system incorporating two magnet systemsconstructed according to the principles of the present invention isindicated generally as 200 in FIG. 17. The system 200 includes twomagnets 20 and their respective support and moving structures, eachcontained within a housing 50. The housings 50 are disposed on oppositesides of the patients, so that the operating points of each magnetsystem overlap so that the magnetic fields produced by the two systemsare additive. The system 200 is particularly adapted for cardiacprocedures, and the housings 50 are positioned on opposite sides of thepatient's chest. The system 200 includes a patient support, such apatient bed 202, which may or may not be movable. A C-arm 204 mountsbi-planar imaging equipment for making bi-planar images of the proceduresite, and displaying them on the displays 206. The bi-planar imageequipment includes an imaging beam sources, such as x-ray sources 208,and imaging beam receivers or detectors, such as amorphous silicon lastplates 210, which are substantially unaffected by the presence ofmagnetic fields. The magnets 20 inside the housing 50 can be used tonavigate a magnetic medical device in the patient's head by pivoting themagnet about axis A1 and rotating the magnet about axis A2 to achievethe desired magnetic field to orient a magnetic medical device insidethe patient's head. The bi-planar imaging allows the physician and otherhealth care workers to monitor the orientation and position of themagnetic medical device to navigate the distal end of the magneticmedical device to its desired destination. While the magnet assembly isdesigned to apply a magnet field at the systems' operating point, which,as described above is a point thirteen inches from the front face of thehousing 50, the system preferably allows the application of a magneticfield in virtually any direction in sufficient strength for navigationpurposes, e.g. 0.04 T, anywhere in 7 inch diameter circle thirteeninches from the front face of the housing.

What is claimed:
 1. A magnet assembly comprising a magnet composed of aplurality of segments, each segment having a magnetization directionthat optimizes the magnetic field in a selected direction at anoperating point in front of the assembly and so that the pivoting of themagnet about an axis behind the magnet through an arch of less than 90°causes the magnetic field direction at the operating point to vary by180°.
 2. A magnet assembly comprising a magnet mounted for pivotingabout a first axis spaced from the magnet, and rotating about a secondaxis that is perpendicular to and intersects with the first axis.
 3. Amagnet assembly comprising a magnet mounted for pivoting about a firstaxis spaced from the magnet, and rotating about a second axis that isperpendicular to and intersects with the first axis, the magnetcomprising a plurality of segments each with a magnetization directionsuch that through a combination of pivoting and rotating the magnetprojects a magnetic field in any direction at an operating point spacedfrom the front of the assembly.
 4. The magnet assembly according toclaim 3 wherein operating point is at least 12 inches from the magnetassembly.
 5. The magnet assembly according to claim 3 wherein theassembly projects a magnetic field at the operating point of at least0.04 T in any direction.
 6. The magnet assembly according to claim 3wherein the assembly projects a magnetic field at the operating point ofat least 0.1 T in any direction.
 7. In combination, first and secondmagnet assemblies disposed on opposite sides of a patient, each magnetassembly comprising a magnet mounted for pivoting about a first axisspaced from the magnet, and rotating about a second axis that isperpendicular to and intersects with the first axis, the magnetcomprising a plurality of segments each with a magnetization directionsuch that through a combination of pivoting and rotating the magnetprojects a magnetic field in any direction at an operating point spacedfrom the front of the assembly.
 8. A composite focused field magnetcomprising a plurality of parallel segments of magnet material, eachsegment magnetized in a direction to generally maximize the magneticfield in a selected direction at an operating point in front of themagnet, the magnet having a generally cylindrical front face, and backface substantially conforming to a surface of equal contribution to themagnetic field at the magnet's operating point.
 9. The composite focusedfield magnet according to claim 8 wherein the magnet is adapted to pivotabout a first axis behind the magnet, and to rotate about a second axisperpendicular to and extending through the first axis, and wherein themagnet comprises a side edge, configured so that through the desiredpivoting and rotating of the magnet, the magnet remains within anhorizontally extended cylindrical inclusion zone having a 45 degreebeveled forward edge.
 10. The composite focused field magnet accordingto claim 8 wherein the magnet is mounted for pivoting about a first axispositioned behind the magnet, and wherein the radius of curvature of thegenerally cylindrical front face substantially equals the distancebetween the first axis and the front face.